We reported last year the development of an important new method for measuring the degree of electrical coupling between cells. When two intracellular electrodes are used to measure the spontaneous voltage noise from separate cells in a spheroidal aggregate reconstituted from embryonic chick heart cells, the degree of correlation of the two signals represents a measure of cell coupling. When two spontaneously-beating aggregates are brought into contact, their beats entrain to a common rate within 30-60 minutes. This synchronization is prolonged to several hours after exposure of the aggregates to cycloheximide. The degree of electrical coupling will be measured in the presence and absence of cycloheximide and other inhibitors of synchronization. Coupling will be assessed in terms of beat synchronization, passage of fluorescent dyes, passage of current pulses, and cross-correlation of voltage noise. Ultrastructural changes in junctional membranes will be examined in relation to the formation, separation and breakdown of gap junctions during coupling and decoupling, and this information will be correlated with results from electrical studies. BIBLIOGRAPHIC REFERENCES: DeFelice, L.J. and D.L. Alkon. (1977) Noise spectra from the hair cells of the Hermissenda statocyst. Biophys. J. Soc. Abstr. 17. Abstract 17. Abstract. In press. Nathan, R.D. and R.L. De Haan. (1977) In vitro development of fast Na ion conductance in embryonic heart cell aggregates. Biophys. J. Abstract. In press.